Tukkis is right about mixdowns being the same volume level as the multitrack mix. If that's happening on your system, then something's out of whack.
However, in the meantime, here's a fix. Make your mixdown. Now, when it flips into Edit View, click on Effects>Amplitude>Normalize. Enter a value in the normalize box - something like 99.98%, and then normalize the file. It'll bounce it up to max again.